The present invention concerns metal laminates, in particular metal laminates that may undergo a subsequent forming step and cataphoresis step. It also concerns their use, notably for the manufacture of automotive body parts and in the construction sector.
Metal laminates comprise two outer metal sheets between which is interposed a polymer layer.
Generally, the main advantage of metal laminates with respect to metal sheets is that they allow the reduction of weight while meeting the specifications regarding stiffness. Such an advantage is particularly interesting in automotive applications since it contributes to a reduction of the fuel consumption of the vehicle.
Metal laminates, in particular those used for the manufacture of automotive body parts, have to meet severe requirements regarding the mechanical properties, both during forming and in service. The forming steps are in particular those of deep drawing, embossing, bending and hemming. The metal laminates should show good ductility at low temperatures in order to allow forming at these temperatures and ensure stiffness at the temperatures of service that is between −20 and 80° C.
The metal laminate should however also present a sufficient heat resistance (stiffness at high temperature) in order to allow high temperature treatments, in particular cataphoresis.
Further, the intermediate layer should have sufficient adhesive strength with respect to the outer metal layer so that the metal laminate presents the cohesion strength as required. For example, the specifications of the automotive industry require an adhesive strength, which is between 1 and 5 decaN/cm depending on the use of the piece.
Metal laminates with a polypropylene polymer layer are known, for example from EP 598 428. They are satisfactory in terms of rigidity and forming behaviour. However, these metal laminates do not present a satisfying heat resistance. Indeed, polypropylene has a melting temperature around 160° C., which is insufficient with regard to some subsequent treatment steps.
One of the frequent subsequent treatment steps for metal laminates is the painting by cataphoresis. Cataphoresis implies the exposure of the metal laminates to temperatures between 140 and 220° C. for 15 to 30 minutes in order to cure the applied coating layers.
The melting of the polymer at these temperatures leads to a drop in the tensile modulus of the layer, which is much lower than 0.01 MPa. The laminate then might sag under its own weight, yielding important geometrical deformations of the laminate. Further, the polymer might run and/or shrink at the extremities of the laminate, leading to unacceptable defects.
This problem is overcome by using an intermediate layer comprising a continuous woven fleece of thermoplastic polymer fibres impregnated with a thermoset polymer material. The thermoset polymer material also ensures adherence to the metal sheets. Such laminates present a good formability with a good heat resistance.
These laminates however present some drawbacks due to irregularities of the fleece thickness an, the adhesion to the metal sheets. Further, the microstructure of the textile fleece may be imprinted to the outer natural sheet during drawing. Such a surface appearance of laminate is incompatible with a use for the manufacture of automotive body parts.
Further to these drawbacks, the manufacturing process of these laminates is unsatisfactory because the adhesion step of the pre-impregnated fleece to the outer metal sheets is slow, leading to a low productivity.
The document U.S. Pat. No. 4,690,856 discloses metal laminates comprising an adhesive composition with improved adhesion strength. The composition comprises a polymer component and an aminosilane compound. The polymer component comprises a polyamide and a polyolefin grafted with an unsaturated polar moiety. However, such compositions tend to form a multi-block structure of polyamide—grafted polyolefin, which presents high viscosity and low tensile modulus, in particular at elevated temperatures. Further, the grafted groups tend to react with each other, thus blocking the reactive sites that are not available for reaction with the substrate. It is thus necessary to add aminosilanes to the polymer component, in order to obtain sufficient adhesive strength.